$\dfrac{ 4l - m }{ -2 } = \dfrac{ 10l - 9n }{ 2 }$ Solve for $l$.
Explanation: Notice that the left- and right- denominators are opposite $\dfrac{ 4l - m }{ -{2} } = \dfrac{ 10l - 9n }{ {2} }$ So we can multiply both sides by $-2$ $-{2} \cdot \dfrac{ 4l - m }{ -{2} } = -{2} \cdot \dfrac{ 10l - 9n }{ {2} }$ $4l - m = - \cdot \left( 10l - 9n \right) $ Distribute the negative sign on the right side. $4l - m = -10l + 9n$ ${4}l - {1}m = -{10}l + {9}n$ Combine $l$ terms on the left. ${4l} - m = -{10l} + 9n$ ${14l} - m = 9n$ Move the $m$ term to the right. $14l - {m} = 9n$ $14l = 9n + {m}$ Isolate $l$ by dividing both sides by its coefficient. ${14}l = 9n + m$ $l = \dfrac{ 9n + m }{ {14} }$